1. Field of the Invention
This invention relates to offset compensation of the output of a transducer. The invention is particularly, though not exclusively, applicable to offset compensation in current measurement in the control of a switched reluctance machine.
2. Description of Related Art
Many electrical and electronic systems require transducers for converting a parameter into an electrical signal. For example, electric motors and generators require some means of monitoring current for a variety of well-recognized reasons, such as measurement, control and equipment protection. A simple technique for doing this is to derive a signal indicative of the current from the voltage dropped across a series-connected resistor in accordance with Ohm""s law. This is a simple technique, but it has various drawbacks. Firstly, the resistor will have a non-zero temperature coefficient of resistance. It generates heat by virtue of the current flowing through it, which will distort the reading when it is used at a temperature other than that at which it is calibrated, due to its non-zero temperature coefficient of resistance. Secondly, the resistor must be connected directly in the circuit to be monitored. This alone makes resistance current monitoring impracticable in, for example, power circuits in which the current to be monitored is in a circuit at a high potential with respect to the circuit to which the monitored current signal is to be relayed. Thirdly, connecting the resistor in the monitored circuit may distort the operation of the circuit itself to an unacceptable degree.
The problem in relatively high potential circuits has been addressed by electrically isolating the circuit being monitored from the monitoring circuit itself. However, the need for isolation raises the further problem that the potential across the isolation barrier may change very rapidly. A typical example of this is in semiconductor switching circuits in which rapid changes in voltage in the monitored circuit occur as a result of switching. The large rate of change of voltage with respect to time (dV/dt) in the monitored circuit can cause capacitive current flow, induced across the isolation boundary, creating a further opportunity for corruption of the transducer output signal.
Current transformers (CT""s) are a form of transducer by which a measure of current in a conductor can be derived. They are electrically isolated from the conductor itself and they have found extensive use in the field of electrical power engineering as, for example, monitors in current regulation and protection systems.
A known CT relies on the substantial balance of magneto-motive force (MMF) between primary and secondary windings that would exist in a CT using a high permeability core. Ideally, a zero secondary circuit impedance (xe2x80x9cburdenxe2x80x9d) would mean that this balance condition would be achieved at zero core flux. In practice, however, the non-zero burden dictates that a voltage will be dropped across the secondary winding with the result that the core flux will also be non-zero.
The core flux is proportional to the integral of the secondary voltage. In the case of an alternating waveform, the amplitude of the core flux will therefore be inversely proportional to the frequency of the monitored current. In addition, the finite permeability of a real core requires MMF to drive the flux around the core. Assuming a linear response of the magnetic material of the core, this MMF will be directly proportional to the flux. As the core flux increases, a larger MMF will be needed to support it. Thus, with decreasing frequency the CT core absorbs an increasing proportion of the primary MMF. Therefore, the secondary MMF and the output current must fall.
It has been considered that this fall-off in the lower frequency response of CT""s represents an operating limit on their usefulness. A low frequency CT means both a large core and a low secondary impedance to offer a flat frequency response over a specified working frequency range. In the limit, known CT""s cannot operate at dc (zero frequency) because of the non-zero secondary circuit resistance which is present in practice.
To address the problem of measuring current at low frequencies and at dc, current measuring devices have been developed that rely on the Hall effect. These are responsive to the strength of the magnetic field created by the current to be monitored. They are also often referred to in the art as xe2x80x9ccurrent transformersxe2x80x9d although transformer principles are not involved.
A known current transducer based on the Hall effect uses a Hall-effect device arranged in an air gap in an otherwise toroidal core. The conductor carrying the current to be monitored is arranged to pass through the central aperture of the toroid. The Hall-effect device in the gap measures directly the flux resulting from the introduction of MMF in the core due to the current in the conductor.
While the device is relatively simply constructed, it has some disadvantages. Firstly, the response of the core material is not linear in practice. Secondly, the Hall-effect device also has a non-linear response and displays characteristics which introduce a static offset error into measurements. Furthermore, the small amplitude of the Hall voltage at the output of the device requires relatively large gain amplification which may render the monitoring circuit as a whole unacceptably prone to noise.
In general, the open-loop Hall-effect element tends to exhibit inconsistency in its output offset characteristics. That is, the output can be expressed as (k*I)+c, where c is a non-constant offset term. The value of c may vary significantly from transducer to transducer, and may also vary with time, temperature, supply voltage and other factors. This can be a significant deterrent to using what would otherwise be an attractive, low-cost solution. For example, one manufacturer offers a range of current sensors based on their Hall-effect device, but the output offset voltage of their low-cost unit varies by xc2x110% initially, is proportional to supply voltage and exhibits a temperature coefficient of xc2x10.05% per Kelvin. The initial offset can be trimmed out, but the temperature and supply-dependent offset variations may be less easy to deal with.
Feedback has been used in conjunction with a CT and a Hall-effect element. In this arrangement the problem of the secondary voltage in a CT is addressed by controlling a secondary current with an amplifier having an input which is a negative feedback signal from the Hall-effect element proportional to core flux. The secondary MMF is then independent of burden voltage and can be made to follow the MMF due to the current in the conductor closely by adjusting the product of the gain of the feedback amplifier and core permeability. With very large amplifier gain, the balance between the primary and secondary MMF""s is determined only by the offset null of the Hall-effect element. Core linearity becomes largely irrelevant because the feedback action is always such as to maintain zero flux and thus to balance the MMF""s. The ratio of primary to secondary current is, therefore, determined by the transformer turns ratio only.
Such transducers of the xe2x80x9cflux-nullingxe2x80x9d Hall-effect type have been popular in the electric machine control field (for example on switched reluctance motors and generators) because of their dc response, wide bandwidth and small size. An example of the flux-nulling sensor is one manufactured by LEM s.a. of Geneva, Switzerland. These sensors are non-invasive and electrically isolated from the monitored current. However, they are relatively expensive because they need an accurately zeroed Hall-effect element and fast responding amplifiers.
It is an object of the present invention to provide offset compensation for a transducer or a transducer circuit that is both inexpensive and does not require the complexity of the closed loop solutions referred to above.
According to an embodiment of the invention there is provided a transducer circuit comprising a transducer operable to produce an electrical transducer output signal indicative of the magnitude of a monitored parameter, sampling means for sampling the transducer output signal at a moment corresponding to a known value of the parameter, the sampling means providing an offset signal which is equivalent to the transducer signal at the said moment, and a differencer arranged to receive a subsequent transducer output signal and the offset signal and to produce a compensated output which is the difference between the subsequent transducer output and at least the offset signal.
Embodiments of the invention require knowledge of the moment at which the monitored magnitude of the parameter will be at a known value, or knowledge of the value of the parameter at a known moment. The compensation applied takes account of any deviation in the output from the transducer with respect to the known value of the parameter and adjusts the transducer output automatically.
The invention is particularly useful in situations in which the known value of the parameter is zero (typically zero current for zero output from an ideal Hall-effect element). However, adjustment for non-zero outputs from the transducer corresponding to the known value can be effected by including an adder in the circuit which is arranged in relation to the differencer such that a reference signal corresponding to the said known value of the parameter is added to the compensated output.
Preferably, the sampling means comprises a sample and hold circuit. This is desirably a combination of a switch gating the output of the transducer to a storage capacitor. The output of the transducer is supplied to the capacitor to store the transducer output signal at the said moment.
Alternatively, the sampling means may include an analog to digital converter (ADC) which is operable, in response to an actuating signal at the said moment, to derive the offset signal in the form of a digital word from the transducer signal. In this digital implementation of the invention it is preferable that the digital word is stored in a digital word store which supplies the digital word to the differencer.
In the case of the offset compensation by deriving an offset compensation signal from a non-zero output of the transducer, the adder may be a digital adder to which the reference signal is applied in digital form. The output of the adder may be supplied to a digital-to-analog converter (DAC), providing an analog signal indicative of the addition of the offset digital word and the reference digital word. This may then be applied to the differencer together with the offset compensation signal. In an alternative form, the differencer is a digital differencer which is operable to derive a digital form of compensated output.
The invention extends to a method of compensating for offset in the output of an electrical transducer producing an electrical transducer signal in response to a monitored parameter, the method comprising: sampling the transducer output signal at a moment corresponding to a known value of the parameter, and taking the difference between a subsequent transducer output signal and the sample transducer output signal to produce a compensated output.
The method may include adding a reference signal to the compensated output that corresponds to the correct transducer output for the said known value of the parameter at the said moment. This is particularly useful in those cases where the known value of the parameter is non-zero.
The invention also extends, in a particular form, to a switched reluctance drive system comprising a switched reluctance machine having a rotor and a stator and at least one stator winding, switch means connected with the stator winding and being actuatable to control the energy in the winding, and a controller operably connected to actuate the switch means, the controller comprising timing means from which timing signals are derived for actuating the switch means, and a transducer circuit arranged to monitor the current in the winding which current has a known value during a recurring interval, the transducer circuit comprising a current transducer responsive to current in the winding to produce an electrical transducer output signal indicative of the monitored current, sampling means for sampling the transducer signal at a moment coincident with the known current, the sampling means providing an offset signal which is equivalent to the transducer signal at the said moment, a differencer arranged to receive a subsequent transducer output signal and the offset signal and to produce a compensated output which is the difference between the subsequent transducer output signal and at least the offset signal.
Preferably, in switched reluctance machine control, a cyclically recurring interval of zero-current can be used as the known current. Particular advantage of this zero-current interval is taken in embodiments of the invention because it allows regular offset compensation without recourse to a reference signal input by sampling the output from the current transducer during the zero-current interval.
Preferably, the transducer includes a Hall-effect element. Preferably, a simple circuit can be used including the Hall-effect element, or any other transducer connected in an open-loop arrangement.